(1) Field of the Invention
This invention relates to a webbing for safety belts. Specifically, this invention relates to a webbing for seat belts in vehicles, seat belts in airplanes and safety belts employed when working at elevated spots.
(2) Description of the Prior Art
As prior art techniques relating to webbings for safety belts, reference may be had to U.S. Pat. No. 4,018,960 and Japanese Patent Publication No. 2981/1978 by way of example.
In the webbing for a safety belt (hereinafter called simply "webbing" for the sake of brevity) disclosed in U.S. Pat. No. 4,018,960, the selvedges of the webbing are each constructed in a tubular form and warp threads forming the tubular selvedge is made of threads having a greater elongation at break than warp threads in the other part, namely, the main part. Owing to the tubular structure of the selvedges of the webbing, the selvedges are soft, thereby bringing about an advantage that they are free from the drawbacks of angular webbings, in other words, they can avoid the wearing of the clothing of a man under protection and can protect exposed parts of the man from damages such as cuts. Since threads having a higher elongation at break than the warp threads in the other part, i.e., the main part are used as the warp threads of the selvedges, the cut-off of the selvedges is allowed to take place after the cut-off of the main part.
On the other hand, the webbing disclosed in Japanese Patent Publication No. 2981/1978 makes combined use of high tensile strength-low elongation threads and low tensile strength-low elongation threads. The high tensile strength-low elongation threads are connected in a slack state so that the webbing is allowed to be cut off stepwise owing to differences in cut-off load.
Although the webbings disclosed respectively in U.S. Pat. No. 4,018,960 and Japanese Patent Publication No. 2981/1978 demonstrate excellent performance in their breaking tests upon their inspection as final products, they are accompanied by such drawbacks that when combined with fittings and used as safety belts, their selvedges are abraded due to frictional contact of the webbings to the corresponding bent guides, filaments of threads which form the selvedges are therefore cut off to develop fluffing, the appearances of the webbings are hence impaired, the users of the webbings feel uneasy, and the rewinding of the webbings in their corresponding retractors is also troubled. In the case of the webbing described in U.S. Pat. No. 4,018,960 in particular, each selvedge is in a tubular form. Each tubular selvedge is bent and is collapsed soon or later into two plies, one being a front ply and the other a back ply. When the tubular selvedge undergoes sliding contact with a guide in a bent state, the warp threads in the back ply are loosened and are hence rendered susceptible to cutting-off of their filaments. As a result, the back ply undergoes more fluffing and the rewinding of the webbing into the corresponding retractor is troubled frequently. Namely, the webbing described in U.S. Pat. No. 4,018,960 is insufficient in durability. Of these problems, the fluffing problem can be reduced by increasing the density of the weft threads per unit length. However, this solution leads to new problems that the webbing becomes thicker in its entirety, the rewinding of the webbing itself into the corresponding retractor is troubled more often and a larger retractor is required.